1. Field of the Invention
The present invention relates to an electronic ballast for the operation of a gas discharge lamp.
2. Description of the Related Art
The employment of electronic ballasts for the operation of gas discharge lamps leads to significant energy savings due to reduced ballast losses and improved lamp efficiency. The input of an electronic ballast usually constitutes a high frequency filter connected to the voltage supply network, which filter is connected with a rectifier circuit. The rectified supply voltage from the rectifier circuit is delivered to a smoothing circuit for generating an intermediate circuit voltage, an inverter fed with the intermediate circuit voltage then generates a high frequency a.c. voltage, which is applied to the load circuit with the gas discharge lamp arranged therein. The operation of the lamp with the high frequency a.c. voltage leads to a reduction of electrode losses and to an increase of the light yields in the positive column of the lamp.
The purpose of the smoothing circuit consists primarily in making available the power necessary for the operation of the lamp. In an electronic ballast described in WO 99/34647 A1 a step-up converter is used as smoothing circuit, the controllable switch of which step-up converter is controlled by a control circuit, for regulating the intermediate circuit voltage. Regulation is necessary since the output power of the ballast changes, which automatically affects the intermediate circuit voltage.
A further purpose of the smoothing circuit is to avoid as far as possible that disrupting frequencies arise which could be reflected back into the supply network. Such disrupting frequencies arise for example when phase differences appear between the applied voltage and the current. The regulation is thus effected in the ideal case such that the current is as sinusoidal as possible and has no phase displacement with respect to the voltage.
The above-described step-up converter has, however, the characteristic that before a zero-crossing of the received voltage the current flow suddenly jumps to zero and only resumes after the zero-crossing with a certain time delay. The reason for this is found in the switching capacitances lying above the control switch of the step-up converter. The sudden drop of current to zero is disadvantageous since in this period of time current and voltage are out of phase, whichxe2x80x94as described abovexe2x80x94leads to the undesired harmonics and disruptions. This problem occurs in general with the employment of switching regulators.
It is thus the object of the present invention to provide an electronic ballast which has a further improved limiting of harmonics.
This object is achieved by means of a novel electronic ballast which contains a rectifier circuit connectable to an a.c. voltage source, a smoothing circuit, connected to the output of the rectifier circuit, for generating an intermediate circuit voltage, and an inverter fed with the intermediate circuit voltage, at the output of which inverter a connection for the load circuit containing the lamp is connected. The smoothing circuit is formed by a switching regulator the controllable switch of which is controlled by a control circuit. For this purpose, the control circuit detects the intermediate circuit voltage and generates in dependence thereupon a control signal for the switch.
In accordance with the invention the control circuit is so constituted that it additionally detects the input voltage of the smoothing circuit and extends the on-state times for the controllable switch when the input voltage approaches its minimum value.
The basic insight of the present invention thus lies in that the on-state times of the switch are extended in dependence upon the actual value of the input voltage. It has proved that through this measure the rapid fall-off of the current, and thus the occurrence of harmonics and disrupting frequencies, can be avoided.
Further developments of the invention are described hereinafter.
Thus, an advantageous configuration of the invention consists in that the extension of the on-state times is greater, the lesser is the input voltage. In order to achieve this in a simple manner, a particularly advantageous development of the ballast in accordance with the invention consists in that the control circuit works in a digital manner. For this purpose, the control circuit has an analog-digital converter which converts the detected intermediate circuit voltage and the input voltage into two digital values with a precision of at least two bitsxe2x80x94preferably of twelve bits. Within the control circuit there is then provided a digital regulation circuit which on the basis of the two digital values calculates switching information for the operation of the switch and passes this on to a driver circuit which in turn converts this switching information into corresponding control signals.
The advantage of this digital constitution of the control circuit consists in that the influence of the various operational parameters for the control of the switch can be taken into account significantly more simply than would be the case with a purely analog circuit. For this purpose it can be provided that the control circuit has a memory with a comparison table in which to each value of the input voltage there is allocated a particular time interval by which the on-state time for the controllable switch is extended. Additionally it is to be noted that a conversion of the measured operational parameters into digital values with a precision of at least two bits is definitely required, since for example in the case of a xe2x80x9cquasi digitalxe2x80x9d regulation by means of a simple comparison of operational values with reference values a sufficiently high precision could not be obtained, and therewith in the end also stability for the intermediate circuit voltage could not be attained. In contrast thereto, with the digital regulation in accordance with the invention, a very high stability can be attained, and on the other hand a digital circuit occupies only little space so that the ballast can be made more compact overall.
A further development of the invention consists in that the control circuit in accordance with the invention is employed further also for the control of the inverter and therewith for the operation of the lamp. For this purpose there may be provided a further analog-digital converter which transforms an operational parameter of the load circuit into a digital value, whereby the control circuit calculates switching information for the operation of the inverter on the basis of this digital value. As operational parameter there may be detected for example the lamp voltage and/or the lamp current.